The power to punish heads of state for crimes against humanity is a recent discovery: Cromwell's lawyers managed it with Charles I, but their judges were in due course executed for treason. Napoleon we exiled instead to St Helena, and not even FE Smith and Lloyd George could persuade their allies at Versailles to try the Kaiser for invading Belgium.

Nuremberg created a precedent, but it was not until Augusto Pinochet came to London in 1998 to take tea with Mrs Thatcher that the idea of ending the impunity of political and military leaders seemed possible. In those days it was bitterly controversial: the pope, Henry Kissinger, George Bush Snr, and even Fidel Castro wrote to Jack Straw demanding that he be freed. But today there are no such efforts on behalf of Taylor: international justice is here to stay.

That does not mean it should be welcomed uncritically, or that its principal defect should be overlooked – namely it does not in practice apply to the "big five" powers in the security council, or to their close friends (hence Syria's Bashar al-Assad has thus far escaped indictment because Russia supports him). But justice has its own momentum, and this selectivity will change. The importance of the Taylor decision, for example, is that it creates a precedent for prosecuting those who "aid and abet" by sending assistance to brutal factions in a civil war. President Ronald Reagan's conduct in arming the Nicaraguan Contras, if it occurred again, would be seen as comparable to the conduct for which Taylor was convicted.

Taylor supplied arms, ammunition and money to the rebels (and even the herbs that child soldiers were told to rub on their bodies to protect them from bullets) in return for a share of their spoils. What fixed him with criminal liability was that he provided this assistance at the time he knew, from reading newspapers, that the rebels were committing widespread and systematic atrocities. On this basis, any political or military leader who sends arms or ammunition to the brutal forces in Syria is guilty of aiding and abetting what is clearly a crime against humanity.

The Taylor proceedings are far from over: both prosecution and defence are appealing. The prosecution in fact suffered some serious defeats: it failed to prove beyond reasonable doubt that Taylor was "godfather" (in league with Colonel Gaddafi) of the mass-murdering and mass-mutilating rebel factions, or even that he had joined in their blood-curdling conspiracy. But, according to the court, he knew and he approved and he assisted. It is open to question, however, whether this is enough to convict him of aiding crimes which require "specific intent", such as rape or terrorism, so his appeal may on these counts be upheld.

An example of how international justice still needs improvement is provided by the three-year duration of Taylor's trial (seven months were taken up by his own testimony) and the unacceptable 13-month delay in delivering the judgment. Particularly jarring is the 50-year sentence, which consigns Taylor to prison until his 114th birthday. The US prosecutor, ludicrously, had demanded 80 years. It is a peculiarity of American criminal justice to sentence people to terms that expire long after they will: it is a custom both irrational and cruel which should have no place in international justice.

The UK has offered to house Taylor in its prisons, but not for a sentence of such unconscionable length. The appeal court is likely to reduce it. Taylor was, after all, acquitted of much more serious offences; and for all the horror produced by his money and his munitions, punishment must always be kept in perspective. The real problem for international justice is the diplomats and politicians – in the security council – who refuse to send those who ordered the bayoneting of Syrian children in their homes to the international criminal court.